Fix ARM64 assembly size directive in cpu_work.cpp

The GNU assembler expects `.size symbol, .-symbol`.  The previous
`.size spend_cpu_cycles, spend_cpu_cycles` expression is not a constant
and breaks compilation on AArch64 Linux.  Use the correct form so the
project builds on ARM64.

src/cpu_work.cpp

@@ -55,7 +55,7 @@ asm(".text\n"
     "    b.ne .L_loop\n"    // Branch back if not zero
     ".L_end:\n"             // End
     "    ret\n"             // Return
-    ".size spend_cpu_cycles, spend_cpu_cycles\n");
+    ".size spend_cpu_cycles, .-spend_cpu_cycles\n");
 #endif
 
 #endif